Look at the programs, check the pre-reqs, and apply.

My wife is defending her (terminal) Master's thesis in two days, at the age of 58. It took five years and was a hard slog with a full-time job, but she managed.

I'd start budgeting my time, now. What do I have to give up, to make sure I have research time at the end of the day?

Budget money, as well. I would want to know how my day-to-day costs will change when I'm in school several hours a week.

Getting into the mindset of learning rather than doing will be a huge shift, so you could practice your study skills by researching the universities and their programs for an hour every day without interruption - make sure you can sit down and read for a full hour.

Once you pick a program, use social media to ask around. You might know someone who goes there, maybe they could introduce you to the faculty?

Can you define what you mean by "out of academia" for 20 years? Do you mean that you already have another graduate degree but want to return to a different field, or do you mean you completed undergraduate work only?

The former should give you an easier time.

Talk to people in the field. Try to get hired in a lab to get current experience.

Take a few classes. Apply for programs.

Call a school and ask them. They will have better information on this than a bunch of internet randos.

Where are you based? US? If so, which state? Which field? Are you willing to travel far?

There are huge differences in costs (from free to insanely expensive) and quality (not necessarily correlated with costs). But if you want to do a PhD, it could be a good idea to search out a research environment right away and to use the Master's as an opportunity to familiarize yourself with their work.

Edit: Oh, and be ready to change plans. Maybe it's not so fun after all, and you redecide on the whole PhD thing. Or maybe you change your topic. That's fine. Don't force yourself to write a PhD you're not interested in.

Go to the registrar

So one thing I see people skipping is the requirements. Depending on what your undergrad is and what you want to go for for your masters you may have to take some “redmedial” classes as your degree skills may not transfer or be considered too old. This will extend the amount of time you need to complete your degree.

Also, what is the reason you want to go back for it? It is just the desire to have the degree or is there an end goal you have after it? It may not be cost effective in the long run.

If you take student loans, don't do it.

They are cancer and a lot of "academia" is a grift for boomers to live large while taking advantage of people

I think they just opened up next year's FAFSA. Start there. See if you can find some money!

I know someone who was doing that when I was a student. The guy got a scholarship from the unemployment system (basically was allowed to keep his unemployment benefit while attending university) got his master and became a teacher. So it was possible in the 00's.

If you're unemployed talk with your employment advisor.